In certain applications a single line from a telephone central office (i.e. a subscriber loop comprising a TIP lead and a RING lead) is shared between several pieces of telephone equipment. One such application would be a telephone set and an automatic telephone answering machine connected to function from the same central office line. Another application is in an electronic key telephone system (EKTS) wherein several telephone sets may share (or have access to) a single line to the central office (CO).
In very simplistic terms, a central office interface is generally located on the subscriber's premises and is employed on the basis of one central office interface circuit per each piece of telephone equipment employed on a common (i.e. shared) central office line. The central office interface is generally designed to appear electrically, to the central office, as a typical telephone set when "off-hook". It is sometimes designed to isolate the DC component of the signal from the AC component of the signal at the subscriber's end.
An important consideration is to ensure that two or more central office interface circuits, when connected in parallel across a common central office line, do not unduly "load-down" the central office line. Expressed in slightly different terms, the idle (or "on-hook") devices sharing the central office line should not draw speech or signalling currents from the central office line at the expense of the busy (or "off-hook") devices sharing the same central office line.
One means of doing this in the prior art has been to use relay contacts to isolate the central office interface circuit from the central office line when the associated piece of telephone equipment is idle, and to connect the central office interface to the central office line when the associated piece of telephone equipment is busy.
The present invention provides a central office interface circuit which performs the same functions as the prior art interface circuit, but does so without the use of relay contacts. The present invention allows many central office interface circuits (constructed according to the present invention) to be connected in parallel, across a common central office line, with the idle interface circuits being effectively non-active, and drawing essentially no current from the central office line.
Stated in other terms, the present invention is an interface circuit for interconnection between first and second leads of a central office line, and a telephone apparatus, the circuit comprising:
a rectifier means having a pair of input terminals for direct connection to the line, and a pair of output terminals; PA1 a selectively operable switch means, for connection in a series circuit relationship with the telephone apparatus and the pair of output terminals wherein the telephone apparatus includes a transformer having at least one primary winding and one secondary winding, and wherein the series circuit relationship is made with the primary winding; and PA1 a capacitance means for connection in parallel to the switch means such that the capacitance can become charged to a maximum voltage level via the rectifier, when the switch is open, thus impeding the flow of alternating current via the capacitance. PA1 a rectifier means having a pair of input terminals for direct connection to the line, and a pair of output terminals; PA1 a transformer means having a primary winding for selective connection across the pair of output terminals of the rectifier means, and a secondary winding for connection to the telephone apparatus and an impedance, comprising at least a capacitor, connected in parallel to the switch such that the capacitor can become charged to a maximum voltage level via the rectifier, when the switch is open, thus impeding the flow of alternating current via the capacitor. PA1 a rectifier means having a pair of input terminals for direct connection to the line, and a pair of output terminals; PA1 a transformer having a primary winding which is divided into two approximately equal portions, and a secondary winding for connection to the telephone apparatus; PA1 a selectively operable switch means, connected intermediate the two portions of the primary winding and in series circuit relationship therewith, the series combination of the switch means and the two portions responsive to signals appearing at the pair of output terminals of the rectifier means, the switch means selectively serving to establish an electrical connection between the primary winding and the rectifier means; and an impedance, comprising at least a capacitor, connected in parallel to the switch such that the capacitor can become charged to a maximum voltage level via the rectifier, when the switch is open, thus impeding the flow of alternating current via the capacitor.
Stated in still other terms, the present invention is an interface circuit for interconnection between first and second leads of a central office line, and a telephone apparatus, the circuit comprising:
Stated in yet other terms the present invention is an interface circuit for interconnection between first and second leads of a central office line, and a telephone apparatus, the circuit comprising: